MRI-guided HIFU therapy devices are sometimes calibrated by service engineers on the field. One major calibration factor is the ultrasonic power output at different settings of the device. Calibration may be called for e.g. in case some components in the electrical chain of ultrasound (US) generation are changed, the transducer itself is replaced, or the built-in QA self-test indicates a discrepancy.
The gold standard measurement of ultrasound power output is the radiation force balance (RFB) method. This is the test performed in factory calibration of the device, using an accurate and sensitive laboratory balance. However, once the therapy device is installed, the cabling will not allow the therapy bed (and its ultrasound output window) to reach very far from the MR device and its stray B0 field. Laboratory balances rely on a magnetic feedback balancing method, and are unsuitable for measurement in these conditions. A radiation force method is known from the paper ‘A buoyancy method for the measurement of total ultrasound power generated by HIFU transducers’ by A. Shaw in Ultrasound in Med. and Biol 34 (2008)1327-1342. This known method relies on the principle that a due to reflection or absorption of travelling ultrasound waves on a target, a force is exerted on the target equal to the change in momentum flux associated with the wave.